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As the detection of urban active faults becomes increasingly important, high resolution detection of urban blind active faults is very important for urban planning, land use and disaster risk reduction. However, it is difficult to determine the corresponding surface positions in the city environment for noise and building restrictions. The active source reflection seismic technique is considered the best technique to image faults with a high resolution and deep penetration. However, urban geophysical exploration must often consider the complex urban environment, which includes moving vehicles, dense power grids, and irregular buildings. These features make active source reflection seismic exploration difficult for wide application due to its drawbacks of high costs and the necessary use of explosives. In contrast, ambient noise seismic surveys have the advantages of continuous ambient noise sources, low costs and fast deployment. These advantages are good for urban exploration. Although ambient noise seismic surveys have a lower resolution than active seismic surveys, their ultra-high density layout can improve the resolution. We conducted two active source seismic lines and two ambient noise seismic lines near the Huangzhuang-Gaoliying fault (HGF) in a northern suburb of Beijing. The autocorrelation and cross-correlation results are consistent with the active source reflection seismic results. They revealed the location of the HGF, which is composed of a set of steep dip faults. The study of the combination of the two techniques demonstrates that ambient noise seismic surveys are effective for urban active fault detection, especially for larger scale area surveys, and active source reflection seismic surveys can be used for detailed surveys. The combination of the two techniques has a higher efficiency and lower costs and can be widely used in blind urban active faults surveys.

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Active fault detection has an important significance for seismic disaster prevention and mitigation in urban areas. The high-density station arrays have the potential to provide a microtremor survey solution for shallow seismic investigations. However, the resolution limitation of the nodal seismometer and small-scale lateral velocity being inhomogeneous hinder their application in near-surface active fault exploration. Distributed acoustic sensing (DAS) has been developed rapidly in the past few years; it takes an optical fiber as the sensing medium and signal transmission medium, which can continuously detect vibration over long distances with high spatial resolution and low cost. This paper tried to address the issue of near-surface active fault exploration by using DAS. We selected a normal fault in the southern Datong basin, a graben basin in the Shanxi rift system in north China, to carry out the research. Microtremor surveys across the possible range of the active fault were conducted using DAS and nodal seismometers, so as to obtain a shallow shear wave velocity model. Meanwhile, we applied a Brillouin optical time domain reflectometer (BOTDR) and distributed temperature sensing (DTS) to monitor the real-time fluctuation of ground temperature and strain. Our results show that the resolution of the deep structures of the fault via the microtremor survey based on DAS is lower than that via the seismic reflection; whereas, their fault location is consistent, and the near-surface structure of the fault can be traced in the DAS results. In addition, both the BOTDR and DTS results indicate an apparent consistent change in ground temperature and strain across the fault determined by the DAS result, and the combination of surface monitoring and underground exploration will help to accurately avoid active faults and seismic potential assessment in urban areas.